This invention relates to improvements in the production of alkaline catalysed phenol-aldehyde condensates of the kind used in bonding together fibres such as mineral wool or glass fibres, and to binder compositions based on such condensates. The invention also relates to bonded fibre products and to processes for producing such products in which the binder composition used is produced by the novel process of the present invention.
One conventional process for forming fibres of glass or other heat-softenable material, called the rotary process, involves delivering heat-softened or molten glass into a hollow spinner or centrifuge provided with a comparatively large number of orifices in the peripheral wall of the spinner. High speed rotation of the spinner delivers the softened or molten glass through the orifices by centrifugal force. Bodies, streams or primary filaments of glass are produced which are engaged by an annularly-shaped gaseous blast and thereby attenuated into fibres which are entrained in the blast in the form of a hollow beam or column of fibres.
In the fibre-forming operation it has been a usual practice to deliver or apply an uncured binder, such as a phenol-formaldehyde condensate in solution or dispersed form, to the newly attenuated fibres at a region below the attenuating region so that the fibres are sprayed with the uncured binder. Fibres of this character are collected in a mass upon a moving conveyor. The thickness of the mass is controlled to provide a fibrous mat which is conveyed or passed through an oven or curing zone for setting the binder in the mat.
The descending fibres at the region of application of the binder are at a temperature of 500.degree. to 600.degree.F or more even though the zone of application of the binder onto the fibres is substantially below the attenuating region. The prior art suggests that cooling of the fibres may be accomplished by spraying the attenuated fibres with a vaporizable medium, such as water, prior to the application of the binder resin. Volatilization of the water into steam and subsequent discharge of the steam into the atmosphere is either non-objectionable or easily reducible. However, even when the binder is applied to fibres at such a lowered temperature, there is appreciable vaporization of the volatile organic constituent of the binder. These organic vapors when cooled condense into a plume (liquid droplets) which is similar to the mechanism of water vapor condensation into a steam plume. Although the effluent may be washed and filtered, at least some of the volatile material and some binder particles or solids are discharged into the atmosphere through a discharge stack connected through a suction blower arrangement beneath the region of collection of the fibres on the conveyor. As much as 30 percent or more of the binder has been lost in the past through volatilization during application and curing. Because of environmental considerations, discharge of this vapor into the atmosphere is objectionable. Use of the binder of the present invention provides a reduction in the free phenol in the solids discharge stack and wash water, due to reduction in the quantity of phenol used to obtain the same binder efficiency and, we believe, due to the presence of the lignosulphonate particularly when added to neutralise the catalyst.
The preparation of a binder composition for application e.g. to glass fibres in the manner described above has been a subject of a series of developments over the period since about 1945. As disclosed in e.g. U.S. Pat. No. 3,704,199 for many years, the alkaline catalyst used in forming the A stage or resole resin was a strong alkali such as sodium hydroxide or potassium hydroxide. It was necessary to neutralise the alkaline catalyst after the resole had been formed to avoid the resole advancing to the final resite or infusible stage, and this resulted in the formation of water-soluble salts. Such salts were believed to cause a deterioration in the final production particularly under humid conditions due to a fall in the strength of the binder. The system developed originally to combat this problem was to remove the salts of neutralisation, e.g. by an ion-exchange treatment. That is described e.g. in U.S. Pat. No. 2,758,101.
It was then found that one could avoid the use of materials likely to produce free sodium or potassium ions in the product by utilising as a catalyst barium hydroxide. The process utilising barium hydroxide is disclosed e.g. in U.S. Pat. No. 3,704,199. As indicated in that specification such a catalyst when neutralised with sulphuric acid forms particles of barium sulphate which when left in situ does not have any deleterious effect on the weathering properties of the glass fibre product.
As indicated above, the application of the binder is carried out under conditions where not all the material leaving the spraying equipment is actually utilised on the product. The actual level of binder required on the product can in some cases be as low as 3% and can in certain products approach 30%. Thus the binder in terms of raw materials cost forms a substantial part of the raw materials cost, therefore reduction in raw materials cost, along with improved efficiency of application of binder are desirable goals. Any change in binder composition must of course be made without any fall below a desired standard in the effectiveness of the performance of the binder. Our earlier U.K. Patent Specification Nos. 1,316,911 and 1,293,744 describe how it is possible to utilise lignosulphonates as binder extenders, in addition to their function with urea in obtaining a controlled setting time for the binder. Utilisation of lignosulphonates in the manner described in these specifications has enabled considerable cost savings to be made by reduction in the quantity of phenol used.
With increasing shortages of materials and rising costs, and with increased demand for insulation materials so as to conserve energy, it is desirable that the cost of the binder applied to the fibre should be reduced, or the rise in cost kept as low as possible. This is particularly important in the case of glass fibre wool mats which are used for domestic and industrial insulation. At least two of the raw materials which have been almost standard in their use to produce a high grade product, phenol and barium hydroxide, have been the subject of major changes in cost and availability. One approach to reducing cost is therefore to reduce the use of phenol and this has been done as described in U.K. Patent Specification Nos. 1,316,911 and 1,293,744. A further approach would be to utilise cheaper grades of phenol and/or cheaper and more readily available catalysts such as sodium hydroxide and calcium hydroxide.
The use of cheaper grades of phenol such as those obtained from tar distillates means that small quantities of materials such as cresols are present in the raw material. Their presence in the condensate when diluted to form the binder when such a source of phenol is used in the conventional process results in the cresol condensates with formaldehyde separating and interfering with the properties of the final binder. As indicated above, sodium hydroxide has only been used in conjunction with a further process for the removal of the salts formed during the neutralisation stage. The precipitation of calcium hydroxide with sulphuric acid results in the formation of particles with a size in the range up to 20 microns, such particles cannot be allowed to remain in the final product at that size and must be separated introducing a further expensive stage into the preparation of the resin. U.K. Patent Specification No. 1,285,938 suggests that the precipitation of calcium ions with sulphuric acid, phosphoric acid or their ammonium salts can only be accomplished with dilute solutions of these acids or their salts, and that this results in the production of large volumes of dilute resins which are uneconomical in industrial practice. That specification suggests the utilisation of a buffering effect to solve the problem i.e. the use of an alkaline solution of a soluble acidic ammonium salt having an anion which forms an insoluble salt with calcium.
We have now found that we can considerably reduce our binder costs by either utilising a cheaper catalyst such as sodium hydroxide or calcium hydroxide in conjunction, if desired, with a cheaper source of phenol or, if desired, continue to use barium hydroxide with a cheaper source of phenol. We have found that we can operate in these ways while avoiding the problems of the prior art i.e. weathering in the case of sodium hydroxide, particle separation in the case of calcium hydroxide and separation of cresol condensates when using cheaper phenol sources by wholly or partially replacing the organic or mineral acids used to neutralise the resin at the end of the A stage condensation by an acidic lignosulphonate or a liquor containing such a material. We have thus discovered how to utilise materials previously believed to be impossible to use without accepting a considerable deterioration in product quality. We can achieve a product quality within a range acceptable in the marketplace using a cheaper and more readily available catalyst either with synthetic phenol or a phenol source containing impurities. The use of lignosulphonates when applied to neutralise the catalyst either at the end of the formation of the condensate before dilution to form the binder, or during the dilution, also appears to reduce the loss of resin during the subsequent application of the binder composition to the fibre giving increased efficiency of application. Such efficiency is measured by determining the binder solids sprayed and the binder retained on the product, and calculating the percentage retained.
Utilising lignosulphonates in the manner described in this specification, we obtain improved efficiencies of the order of 80% compared with around 62% previously obtained in the conventional process, or around 72% when lignosulphonates are simply added at the binder mixing stage to a resin neutralised by sulphuric acid.
The term "lignosulphonate" is used to refer to the material produced as a by-product in the digestion of wood pulp. During this digestion with an inorganic bisulphite, lignosulphonates are formed, and some of the hemi-cellulose is converted to carbohydrates. The liquor formed may be spray-dried to give a solid material, or concentrated to a liquor of a particular solids concentration. In some cases, a purer material is formed by separating the carbohydrate or sugar material from the crude liquor. The lignosulphonate in liquor form is also known as "waste sulphite lye". Lignosulphonates can be obtained readily in solid or liquid form, and are derived from the use of one of the following bisulphites: ammonium, calcium, magnesium and sodium. The lignosulphonate when dispersed in water will normally give an acid pH, and can therefore be used in the neutralisation of the alkaline catalyst in the aqueous solution of a phenol-formaldehyde condensate. Some lignosulphonates formed by the use of sodium bisulphite are in fact alkaline and cannot be used in the process of this invention.
The assessment as to whether a particular lignosulphonate source can be used as a neutralising agent will of course depend on its acidity. This can be measured in terms of the acid equivalent, in other words the quantity of SO.sub.3 H groups present in the sulphonated molecules e.g. the reported acid equivalent value for a commercial source of ammonium lignosulphonate sold under the trade name Totanin is 400 to 500. We find that we can select suitable materials by measuring the pH value of solutions of the lignosulphonate material made up so as to contain 10% solids. The values obtained will vary according to the source, and in some cases, with changes in the nature of the wood being pulped. It is clearly impossible with a natural raw material made under varying conditions to define completely all the possible variations but as a general guide, we find it preferable to select a source which provides a material which in the form of a solution containing 10% solids has a pH in the range 3.5 to 4.5. This does not mean that materials outside this range cannot be used as long as they are acidic, but problems may be encountered in combating the effect of either too little or too much lignosulphonate material in the final binder composition, with a resultant effect on the binder setting time.